Vinyl acetate is conventionally produced via a vapor phase reaction of ethylene, oxygen and acetic acid, e.g., the acetoxylation of ethylene. The reaction is typically conducted in a fixed bed catalyst reactor. The catalyst may comprise palladium or a palladium/gold mixture, which is supported on a silica or alumina base. In addition to the formation of vinyl acetate, the undesired combustion of ethylene to form carbon dioxide and water also takes place. Other undesired impurities that may be formed include acetaldehyde, ethyl acetate, methyl acetate, acetone, ethylene glycol diacetate, acrolein and crotonaldehyde.
The selectivity and conversion relating to the reaction are functions of several variables including reactor temperature, component concentration, and the condition of the catalyst. Deactivation of the catalyst, which routinely occurs over time due to buildup of tars and polymeric materials on the catalyst surface and/or to structural changes of the catalyst metals, can adversely affect the reaction process, particularly with regard to selectivity. These changes in reactor performance can ultimately lead to compositional changes in the liquid stream entering the purification section of a vinyl acetate plant.
The reaction yields a crude vinyl acetate product comprising vinyl acetate, water, and carbon dioxide as well as unreacted ethylene and acetic acid, which are used in excess. The ethylene and acetic acid are recycled back to the reactor from the reaction and purification sections of the unit. Product vinyl acetate is recovered and purified in the purification section and sent to storage tanks. Wastewater is sent to a treatment facility and carbon dioxide is vented to a pollution control device. Inert gases such as nitrogen and argon may accumulate over time and may then be purged from the reaction section to minimize buildup.
Generally speaking, the rate of acetoxylation increases as the concentration of oxygen in the reactor is increased. However, the amount of oxygen that may be introduced into the reactor is limited by a flammability limit of the reaction mixture. The flammability limit is typically defined as the lowest concentration of oxygen in a mixture that will result in a pressure rise when it contacts an ignition source. If the oxygen concentration exceeds this flammability limit, a fire or explosion could result.
Various steps have been taken to minimize the risk of such a fire or explosion. For example, in the fixed-bed reactor of EP 0 845 453, the concentration of oxygen in the inlet gas composition is closely monitored and maintained at or near a threshold value. The mathematical approximations used to define this threshold value are described in EP 0 845 453 which is incorporated herein by reference. When the inlet oxygen concentration exceeds this threshold value, a shut-down signal is activated, and the reaction is quenched by shutting off the ingress of fresh oxygen into the reactor.
The conventional calculation of flammability limits and/or the establishment of non-flammability ranges, however, may be inherently inaccurate. The conventional experimental techniques and methods that are used to develop mathematical correlations generally calculate flammability limits that are low. These correlations provide a buffer from the true flammability limit. Although safety is achieved, reaction efficiencies suffer due to operation at lower oxygen concentrations.
Even in view of the conventional processes, the need exists for a vinyl acetate production process that calculates more accurate flammability limit correlations, which provide for safe control of the process and improved operational efficiencies.
The references mentioned herein are hereby incorporated by reference.